1. Field of the Invention
An aspect of this disclosure relates to an image forming apparatus.
2. Description of the Related Art
In the commercial printing industry, for small-lot printing of various types of data and variable data printing, a Print On Demand (POD) system including an electrophotographic image forming apparatus has become more popularly used than an offset press. An electrophotographic image forming apparatus used for such a purpose needs to provide accurate registration or register (the correspondence of the position of printed matter on the two sides of a sheet) and image uniformity that are comparable to those of an offset press.
Causes of misregisteration or misregister (i.e., inaccurate registration) in an image forming apparatus include registration error in the vertical or horizontal direction, skew error between a recording medium and a printed image, and change in image length caused when a toner image is transferred. Also, in an image forming apparatus including a fusing unit, misregistration may occur due to an image magnification error that is caused when a recording medium heated by the fusing unit expands or contracts.
In a related-art technology for preventing misregistration, after an image is printed on a front surface of a paper sheet (an example of a recording medium), dimensions of the paper sheet in the main-scanning and sub-scanning directions are detected at given positions on the paper sheet, and the magnification of an image to be printed on a back surface of the paper sheet is corrected based on changes in the size of the paper sheet that are determined based on the detected dimensions of the paper sheet (see, for example, Japanese Laid-Open Patent Publication No. 2004-271739 and Japanese Laid-Open Patent Publication No. 2007-102090).
Here, when a paper sheet is heated and pressed by a fusing unit to print an image on the front surface, the shape of the paper sheet unevenly changes, for example, from a rectangle to a trapezoid. That is, a paper sheet is deformed unevenly in the main-scanning direction and the sub-scanning direction. However, with the related-art technology where changes in the size of a paper sheet are determined based on the dimensions of the paper sheet in the main-scanning direction and the sub-scanning direction, each of which is detected at one position on the paper sheet, it is not possible to detect changes in the size of the paper sheet at other positions and therefore it is difficult to accurately determine the shape of a deformed paper sheet.